The bacteriophage T4 is a powerful tool in the analysis of basic mechanisms of mutagenesis. The genetics of the rII region have been extensively used and inferences of particular mutagenic pathways can be made. With molecular cloning and DNA sequencing techniques we can now confirm certain inferred pathways. Cloned sequences will also aid in the biochemical analysis of certain T4 functions involved in fidelity and repair. The genome of bacteriophage T4 will be cloned by recombinant DNA techniques. First our efforts have been developing new techniques for cloning of T4 DNA without making C-containing DNA. The primary cloning vector is M13. The cloned sequences have been probed for rII regions of interest by DNA:DNA hydridization. These regions have been sequenced.